The majority of signals received by an electronic device such as a mobile phone are analog signals. An analog signal is a signal having a continuous range of analog signal levels. On the other hand, the processor of the electronic device can only process digital data. Therefore, an intermediate device is required to convert analog signals into a binary form of ones and zeros for digital processing. Such an intermediate device is called an analog-to-digital converter (ADC). There are a variety of ADC architectures including pipelined, flash, Sigma-Delta, successive approximation and the like.
An n-bit flash ADC comprises five parts: a reference voltage generator, a track-and-hold (TH) amplifier, a comparator array, a latch device array and an encoder. In the n-bit flash ADC, a reference voltage generator is typically formed by 2n resistors connected in series between a Ref+ voltage and a Ref− voltage to produce 2n−1 reference voltage levels for respective reference inputs of the comparator array. The comparator array comprises 2n−1 comparators, each of which receives an analog signal via the TH amplifier and a reference voltage from the reference generator. Each comparator generates a digital number based upon the sign of the difference between two inputs. Such a digital number is sent to the encoder via a latch device array. The encoder generates an n-bit binary code, which can be processed by digital devices, such as digital signal processors, central process units, microcontrollers or the like.
In an ADC, some errors such as comparator offsets and amplifier nonlinearity errors may affect the accuracy of the ADC. These errors need to be addressed for the optimum performance of the ADC. To address the issues of offsets and nonlinearity in Flash ADC different architectures based on averaging and interpolation, pre-distorted reference and a combination of both have been proposed and tried. While the first type can compensate comparator offsets only, the second type can take care of offset or non-linearity but not both. The third technique, though proposed to compensate both offsets and nonlinearity, is not effective to compensate either. Moreover, most of the techniques require an external calibration input.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.